1. Field of the Invention
The present invention relates to a liquid crystal display device to be subjected to counter-electrode voltage adjustment work, and more particularly to a technology of flicker adjustment for an active matrix type liquid crystal display device using a remote controller.
2. Description of the Related Art
Generally, an active matrix type liquid crystal display (hereafter referred to simply as “AM-LCD”) is often used for a liquid crystal display device such as an LCD television receiver or an LCD projection television receiver. The AM-LCD device is typically formed such that a liquid crystal layer is sandwiched between a pair of substrates to be a liquid crystal panel, that a plurality of gate lines and a plurality of source lines are formed on one of the substrates, and that a TFT (Thin Film Transistor) is provided as a switching element at each intersection between each gate line and each source line for each pixel of the liquid crystal panel. When a gate voltage is applied to a gate electrode of a TFT for a pixel, which is assumed here as a P-type FET (Field Effect Transistor), so as to turn on the TFT, a voltage based on a video signal from a source line is applied to a drain electrode of the TFT and is in turn applied to a pixel electrode (display electrode) for the pixel on the one substrate of the liquid crystal panel. On the other hand, a counter-electrode voltage is applied to a counter-electrode provided on the other substrate for the pixel so as to drive the liquid crystal of the pixel, for display, by a voltage difference between the drain voltage and the counter-electrode voltage for the pixel.
Note here that AC (Alternate Current) drive is used, rather than DC (Direct Current) drive, to drive the liquid crystal for the following reason. If a liquid crystal is driven by a DC current, ions stay on one side of the liquid crystal, which causes an after-image on the display. This reduces the display quality, and significantly degrades the property of the liquid crystal, which sometimes causes a burned-in image. Thus, AC drive is used as a method of driving the liquid crystal, considering the life of the liquid crystal material.
However, a TFT liquid crystal panel has a TFT parasitic capacitance, so that a feed-through voltage VF expressed by the following equation is superimposed as a DC component on the pixel electrode voltage to drive the liquid crystal:VF=−VGCGS/(CGS+CLC+CSC)where VG is gate pulse voltage, CGS is parasitic capacitance between the gate electrode and source electrode of the TFT, CLC is liquid crystal capacitance formed by the liquid crystal between the counter-electrode and the drain electrode, and CSC is auxiliary capacitance provided between the drain electrode and the gate line. Since this feed-through voltage VF, as a DC component, degrades the liquid crystal, it is required to reduce an apparent value of the DC component to substantially zero e.g. by biasing the counter-electrode voltage or the voltage based on the video signal with a voltage corresponding to the feed-through voltage VF.
Furthermore, the liquid crystal capacitance CLC varies with a magnitude of the voltage due to the dielectric constant anisotropy of the liquid crystal, and consequently the DC component also varies. Accordingly, it is not possible to reduce the DC component to zero for the full range of gradation. In addition, when a drive signal for the AC drive becomes asymmetric due to the superimposition of the DC component, flicker occurs at the same period as that of the frequency of the driving signal. It is known that when a video input voltage applied to a liquid crystal in an LCD television display device is asymmetric relative to the counter-electrode voltage, it causes 30 Hz flicker visible to a human eye, because the frequency of a driving signal in an LCD television display device is ½ of a frame frequency. For reducing the flicker, some AC drive modes of driving video signals are available which use field inversion, line inversion or a combination of these. However, it is difficult for any of such available drive modes to completely reduce the flicker, so that some additional adjustment for reducing the flicker is needed.
For this reason, a conventional TFT-LCD device adjusts the counter-electrode voltage so as to minimize the flicker to improve the image quality of the LCD device, and at the same time to prevent degradation of a liquid crystal material due to the DC component. There, the adjustment to minimize the flicker is made by adjusting the counter-electrode voltage. Actually, an operator (human) adjusts the counter-electrode voltage while viewing and inspecting the state of flicker.
FIG. 7 is a schematic block diagram of a conventional LCD television receiver 100 to be subjected to the counter-electrode voltage adjustment. Referring to FIG. 7, the LCD television receiver 100 comprises a television receiver body 110, an antenna 102, an LCD 103, a speaker 104 and a remote controller 105. The television receiver body 110 includes a counter-electrode voltage adjustment unit 107 as will be described in detail later.
The television receiver body 110 comprises a tuner 111 to be commanded by the remote controller 105 for selecting a channel as well as a video decoder 112 and an audio decoder 113 for decoding a video signal and an audio signal. The television receiver body 110 further comprises a video output circuit 114 and an audio output circuit 115 for outputting the video signal and the audio signal to the LCD 103 and the speaker 104, respectively, for display and sound. The television receiver body 110 still further comprises a control unit 116 formed of a microcomputer for controlling the tuner 111, the video decoder 112, the audio decoder 113, the video output circuit 114 and the audio output circuit 115. The control unit 116 is coupled to a memory 117 in the television receiver body 110 to store various data needed for the controls. The video output circuit 114 applies a voltage based on the video signal to a display electrode 131 of the LCD 103, while the counter-electrode voltage adjustment unit 107 applies a counter-electrode voltage to a counter-electrode 132 of the LCD 103. By adjusting the counter-electrode voltage, flicker is reduced.
FIG. 8 is a circuit diagram of the counter-electrode voltage adjustment unit 107 of the conventional LCD television receiver 100. The counter-electrode voltage adjustment unit 107 comprises: an operational amplifier A; a resistor circuit composed of resistors R2 and R3 and a three-terminal variable resistor Rx connected in series between a constant DC power supply voltage Vo and ground; and an emitter-follower transistor Q1. The operational amplifier A has, on its input side, a positive (+) input connected to ground, and a negative (−) input supplied with an adjustment voltage Vr which is generated by a resistance division of the resistor circuit of the resistors R2, R3 and Rx. On the other hand, the output of the operational amplifier A is taken from the emitter of the emitter-follower transistor Q1 having an emitter resistor R1, and is smoothed by a capacitor C1 connected between the emitter and ground, and is further output to the LCD 103 as a DC counter-electrode voltage Vcom (DC). Note here that the DC counter-electrode voltage Vcom (DC) shown in FIG. 8 is converted by a DC-to-AC converter (not shown) to an AC counter-electrode voltage Vcom (AC) to be applied to the counter-electrode 132 of the LCD 103.
For operating the counter-electrode voltage adjustment unit 107 to adjust the counter-electrode voltage, an operator (human) turns a knob of the variable resistor Rx with a screwdriver, while viewing a display screen of the LCD 103, so as to adjust the resistance of the variable resistor Rx. This causes the following problem. The variable resistor Rx is mounted on a printed circuit board which is placed in the LCD television receiver 100 on a side opposite to the front of the display screen. Thus, the front of the display screen is distanced from the variable resistor Rx. Accordingly, it is difficult for the operator to turn the knob of the variable resistor Rx for the adjustment with its hand stretched out while viewing the display screen. This is particularly so when the display screen is large-sized, so that the adjustment is difficult.
To solve this problem, a mirror is sometimes used to view the display screen for the adjustment. However, an adjustment work using a mirror is not easy, particularly when the display screen is large-sized. With a large-sized display screen and a mirror, it is significantly difficult to view and inspect the flicker on the display screen, resulting in a long time for the adjustment and reduction of adjustment accuracy. In addition, the variable resistor Rx reduces its reliability if turned many times for the adjustment.
There are other methods of adjusting a counter-electrode voltage in an LCD device. For example, Japanese Patent 3058049 discloses an LCD device which automatically adjusts flicker by using a wired remote controller with an optical sensor having a light receiving unit including a phototransistor. In this LCD device, the optical sensor is directed to a projection screen so as to detect flicker by an intensity of light from the projection screen. More specifically, the LCD device detects 30 Hz flicker using the light receiving unit of the optical sensor, a low-pass filter, a peak hold circuit, a timing generator and so on. The detected flicker is subjected to data processing using a data processing circuit having a register, a comparator and the like to obtain a processed data, which is then used to adjust a counter-electrode voltage in the LCD device so as to automatically adjust the flicker.
However, this LCD device requires a wired remote controller to be exclusively used for the counter-electrode voltage adjustment, and also requires many circuit elements for the flicker detection, data storage, data processing and so on for the purpose of the automatic adjustment. This results in increased circuit complexity and increased cost, and causes power consumption to increase due to the increased circuit size. In addition to this problem, the LCD device has a further problem that the wired remote controller can move only in a limited range because of the wiring, and is harder to use than an ordinary remote controller by wireless or light transmission.
Japanese Laid-open Patent Publication 2002-202761 discloses an AM-LCD device having a counter-electrode voltage generating circuit which generates, and applies to a counter-electrode, different counter-electrode voltages for a forward scanning and a reverse scanning, respectively. The AM-LCD device separately adjusts, and applies to the counter-electrode, the counter-electrode voltages for the forward and reverse scannings, respectively. For adjusting each counter-electrode voltage, the AM-LCD device uses a variable resistor in the counter-electrode voltage generating circuit mounted in the device. This causes a problem that it is difficult to make the counter-electrode voltage adjustment, particularly for a large-sized display, just as the above-described conventional LCD television receiver.
Besides, Japanese-translated Laid-open Publication of International Patent Application 2004-514947 discloses an LCD device which controls a common electrode (counter-electrode) voltage using two photosensors. More specifically, the two photosensors detect a projection image of a video signal (for pre-adjustment) having preset color components and drive level, and output electrical signals corresponding to light levels which the two photosensors detect, respectively. An electrical signal corresponding to the difference between the two output electrical signals is extracted by a differential amplifier, and is used to adjust the common electrode voltage. This causes a problem that it requires use of multiple photosensors to achieve high detection accuracy, and also requires alignment accuracy of the photosensors, resulting in increased circuit complexity and increased cost.